In recent years, progress has been made in the research and development of organic light-emitting elements, which are light-emitting elements that rely on the phenomenon of electroluminescence of solid fluorescent substance. An organic light-emitting element includes a substrate, an interlayer insulation film, an anode, and a hole injection layer, which are formed in the stated order. A bank made of insulating material is formed to surround a peripheral edge of the hole injection layer. In an opening defined by the bank, a hole transport layer, a light-emitting layer, and an electron transport layer are layered. Furthermore, a cathode is formed on the electron transport layer. In the organic light-emitting element, the structure including the layers from the anode to the cathode corresponds to an element structure.
Manufacturing of the organic light-emitting element includes an aging process in which electric current is applied between the anode and the cathode in the element structure. The aging process is important in order to suppress degradation of the organic light-emitting element over time, to stabilize the light-emitting properties, and to increase the lifetime of the organic light-emitting element. Accordingly, various types of research and development have been conducted on an aging process (e.g., Patent Literatures 1 and 2).